Proton is a wrapped/enhanced version of Wine, which has been a thing forever. A large chunk of the Steam library worked perfectly fine on Linux before Proton.
Source? My recollection was that it didn't. Wine had awful direct x translation, I'm not sure if it could do dx11 at all when the steam machine came out. DXVK is a proton project and without it few games could actually run at all.
That's not fair. Vulkan didn't exist when the original Steam Machines launched. Wine's Direct3D implementation also had different goals than the DXVK project, such as supporting macOS, older hardware, and non-gaming DirectX uses.